Method of making fluorinated electrets

ABSTRACT

An electret is described that includes a plasma surface modified polymeric article having plasma surface fluorination produced by fluorinating the polymeric article.

This is a continuation of U.S. application Ser. No. 10/126,028, filedApr. 19, 2002 now U.S. Pat. No. 6,562,112, which is a continuation ofU.S. application Ser. No. 09/109,497, filed Jul. 2, 1998 now U.S. Pat.No. 6,432,175.

This invention relates to preparing fluorinated electrets.

BACKGROUND

The filtration properties of nonwoven polymeric fibrous webs can beimproved by transforming the web into an electret, i.e., a dielectricmaterial exhibiting a quasi-permanent electrical charge. Electrets areeffective in enhancing particle capture in aerosol filters. Electretsare useful in a variety of devices including, e.g., air filters, facemasks, and respirators, and as electrostatic elements inelectro-acoustic devices such as microphones, headphones, andelectrostatic recorders.

Electrets are currently produced by a variety of methods includingdirect current (“DC”) corona charging (see, e.g., U.S. Pat. No. Re.30,782 (van Turnhout)), and hydrocharging (see, e.g., U.S. Pat. No.5,496,507 (Angadjivand et al.)), and can be improved by incorporatingfluorochemicals into the melt used to produce the fibers of someelectrets (see, e.g., U.S. Pat. No. 5,025,052 (Crater et al.)).

Many of the particles and contaminants with which electret filters comeinto contact interfere with the filtering capabilities of the webs.Liquid aerosols, for example, particularly oily aerosols, tend to causeelectret filters to lose their electret enhanced filtering efficiency(see, e.g., U.S. Pat. No. 5,411,576 (Jones et al.)).

Numerous methods have been developed to compensate for loss of filteringefficiency. One method includes increasing the amount of the nonwovenpolymeric web in the electret filter by adding layers of web orincreasing the thickness of the electret filter. The additional web,however, increases the breathing resistance of the electret filter, addsweight and bulk to the electret filter, and increases the cost of theelectret filter. Another method for improving an electret filter'sresistance to oily aerosols includes forming the electret filter fromresins that include melt processable fluorochemical additives such asfluorochemical oxazolidinones, fluorochemical piperazines, andperfluorinated alkanes. (See, e.g., U.S. Pat. No. 5,025,052 (Crater etal.)). The fluorochemicals should be melt processable, i.e., suffersubstantially no degradation under the melt processing conditions usedto form the microfibers that are used in the fibrous webs of someelectrets. (See, e.g., WO 97/07272 (Minnesota Mining andManufacturing)).

SUMMARY OF THE INVENTION

In one aspect, the invention features an electret that includes asurface modified polymeric article having surface fluorination producedby fluorinating a polymeric article. In one embodiment, the articleincludes at least about 45 atomic % fluorine as detected by ESCA. Inanother embodiment, the article includes a CF₃:CF₂ ratio of at leastabout 0.25 as determined according to the Method for DeterminingCF₃:CF₂. In other embodiments, the article includes a CF₃:CF₂ ratio ofat least about 0.45 as determined according to the Method forDetermining CF₃:CF₂.

In one embodiment, the article has a Quality Factor of at least about0.25/mmH₂O, (preferably at least about 0.5/mmH₂O, more preferably atleast about 1/mmH₂O).

In some embodiments, the article includes a nonwoven polymeric fibrousweb. Examples of suitable fibers for the nonwoven polymeric fibrous webinclude polycarbonate, polyolefin, polyester, halogenated polyvinyl,polystyrene, and combinations thereof. Particularly useful fibersinclude polypropylene, poly-(4-methyl-1-pentene), and combinationsthereof. In one embodiment, the article includes meltblown microfibers.

In another aspect, the invention features an electret that includes apolymeric article having at least about 45 atomic % fluorine as detectedby ESCA, and a CF₃:CF₂ ratio of at least about 0.45 as determinedaccording to the Method for Determining CF₃:CF₂. In another embodiment,the electret includes at least about 50 atomic % fluorine as detected byESCA, and a CF₃:CF₂ ratio of at least about 0.25 as determined accordingto the Method for Determining CF₃:CF₂.

In other aspects, the invention features a respirator that includes theabove-described electrets. In still other aspects, the inventionfeatures a filter that includes the above-described electrets.

In one aspect, the invention features a method of making an electretthat includes: (a) fluorinating a polymeric article to produce anarticle having surface fluorination; and (b) charging the fluorinatedarticle in a manner sufficient to produce an electret. In oneembodiment, the method includes charging the fluorinated article bycontacting the fluorinated article with water in a manner sufficient toproduce an electret, and drying the article. The method is useful formaking the above-described electrets. In another embodiment, the methodincludes charging the fluorinated article by impinging jets of water ora stream of water droplets onto the fluorinated article at a pressureand for a period sufficient to produce an electret, and drying thearticle.

In other embodiments, the method includes fluorinating a polymericarticle in the presence of an electrical discharge (e.g., an alternatingcurrent corona discharge at atmospheric pressure) to produce afluorinated article. In one embodiment, the method includes fluorinatingthe polymeric article in an atmosphere that includes fluorine containingspecies selected from the group consisting of elemental fluorine,fluorocarbons, hydrofluorocarbons, fluorinated sulfur, fluorinatednitrogen and combinations thereof. Examples of suitable fluorinecontaining species include C₅F₁₂, C₂F₆, CF₄, hexafluoropropylene, SF₆,NF₃, and combinations thereof.

In other embodiments, the method includes fluorinating the polymericarticle in an atmosphere that includes elemental fluorine.

In other embodiments, the method of making the electret includes: (A)fluorinating a nonwoven polymeric fibrous web (i) in an atmosphere thatincludes fluorine containing species and an inert gas, and (ii) in thepresence of an electrical discharge to produce a web having surfacefluorination; and (B) charging the fluorinated web in a mannersufficient to produce an electret.

In other aspects, the invention features a method of filtering thatincludes passing an aerosol through the above-described electrets toremove contaminants.

The fluorinated electrets of the invention exhibit a relatively highoily mist resistance relative to non-fluorinated electrets.

Glossary

In reference to the invention, these terms having the meanings set forthbelow:

“electret” means a dielectric material exhibiting a quasi-permanentelectrical charge. The term “quasi-permanent” means that the timeconstants characteristic for the decay of the charge are much longerthan the time period over which the electret is used;

“surface modified” means that the chemical structure at the surface hasbeen altered from its original state.

“surface fluorination” means the presence of fluorine atoms on a surface(e.g., the surface of an article);

“fluorine containing species” means molecules and moieties containingfluorine atoms including, e.g., fluorine atoms, elemental fluorine, andfluorine containing radicals;

“fluorinating” means placing fluorine atoms on the surface of an articleby transferring fluorine containing species from a gaseous phase to thearticle by chemical reaction, sorption, condensation, or other suitablemeans;

“aerosol” means a gas that contains suspended particles in solid orliquid form; and

“contaminants” means particles and/or other substances that generallymay not be considered to be particles (e.g., organic vapors).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plot of % DOP Penetration vs. DOP Load for Examples 36 and37.

FIG. 2 is a plot of % DOP Penetration vs. DOP Load for Examples 38 and39.

FIG. 3 is a plot of % DOP Penetration vs. DOP Load for Example 40.

DESCRIPTION OF PREFERRED EMBODIMENTS

The electret includes a surface modified polymeric article (e.g., anonwoven polymeric fibrous web) produced by fluorinating a polymericarticle. The electrets preferably have sufficient surface fluorinationto provide oily mist resistance. One measure of oily mist resistance ishow well the electret maintains its Quality Factor during challenge withan aerosol. The Quality Factor can be calculated from results obtainedfrom the dioctylphthalate (“DOP”) initial penetration test (“the DOPtest”). The DOP test also provides a relative measure of the chargestate of the filter. The DOP test procedure involves forcing DOP aerosolat a face velocity of 6.9 cm/second for a period of about 30 secondsthrough the sample, measuring the pressure drop across the sample(Pressure Drop measured in mmH₂O) with a differential manometer, andmeasuring the percent DOP penetration (DOPPen %). The Quality Factor(QF) (measured in 1/mmH₂O) can be calculated from these values accordingto the following formula:${{QF}\lbrack {1\text{/}{mm}\quad H_{2}O} \rbrack} = \frac{{- {Ln}}\frac{{DOPPenetration}(\%)}{100}}{{PressureDrop}\lbrack {{mm}\quad H_{2}O} \rbrack}$

The higher the Quality Factor at a given flow rate, the better thefiltering performance of the electret.

Preferred electrets have a Quality Factor of at least about 0.25/mmH₂O,preferably at least about 0.5/mmH₂O, more preferably at least about1.0/mmH₂O.

Electron spectroscopy for chemical analysis (“ESCA”) (also known asX-ray photoelectron spectroscopy (“XPS”)) provides one measure ofsurface fluorination. Preferably the surface of the electret exhibits atleast about 45 atomic % fluorine, more preferably at least about 50atomic % fluorine when analyzed by ESCA. ESCA analyzes the elementalcomposition of the outermost surface (i.e., approximately 10 to 50 Å) ofa specimen. ESCA can be used to detect all elements in the periodictable except helium and hydrogen.

The electret also has a CF₃:CF₂ ratio at the surface of the electret ofat least about 0.25, preferably at least about 0.45, and more preferablygreater than 0.9, as determined according to the Method For DeterminingCF₃:CF₂ ratio set forth in the Example section below.

In one embodiment, the electrets include nonwoven polymeric fibrous websthat include fibers such as, e.g., meltblown microfibers, staple fibers,fibrillated films, and combinations thereof. The fibers can be formedfrom resins. Preferably the resin is a thermoplastic nonconductive,i.e., having a resistivity of greater than 10¹⁴ ohm-cm, resin. The resinused to form the fibers should be substantially free of materials suchas antistatic agents that could increase the electrical conductivity orotherwise interfere with the ability of the fibers to accept and holdelectrostatic charges.

Examples of useful thermoplastic resins include polyolefins such as,e.g., polypropylene, polyethylene, poly-(4-methyl-1-pentene), andcombinations thereof, halogenated vinyl polymers (e.g., polyvinylchloride), polystyrene, polycarbonates, polyesters, and combinationsthereof.

Additives can be blended with the resin including, e.g., pigment, UVstabilizers, antioxidants, and combinations thereof.

Meltblown microfibers can be prepared as described in Wente, Van A.,“Superfine Thermoplastic Fibers,” Industrial Eng. Chemistry, Vol. 48,pp. 1342-1346 and in Report No. 4364 of the Naval Research laboratories,published May 25, 1954, entitled, “Manufacture of Super Fine OrganicFibers,” by Wente et al. Meltblown microfibers preferably have aneffective fiber diameter in the range of less than 1 to 50 μm ascalculated according to the method set forth in Davies, C. N., “TheSeparation of Airborne Dust and Particles,” Institution of MechanicalEngineers, London, Proceedings 1B, 1952.

The presence of staple fibers provides a more lofty, less dense web thana web constructed solely of meltblown microfibers. Preferably theelectret contains more than 70% by weight staple fibers. Webs containingstaple fibers are disclosed in U.S. Pat. No. 4,118,531 (Hauser).

Electrets that include a nonwoven polymeric fibrous web preferably havea basis weight in the range of about 10 to 500 g/m², more preferablyabout 10 to 100 g/m². The thickness of the nonwoven polymeric fibrousweb is preferably about 0.25 to 20 mm, more preferably about 0.5 to 2mm.

The nonwoven polymeric webs of the electret can also include particulatematter as disclosed, for example, in U.S. Pat. No. 3,971,373, (Braun),U.S. Pat. No. 4,100,324 (Anderson), and U.S. Pat. No. 4,429,001 (Kolpinet al.).

Electret Preparation

The electrets can be prepared by fluorinating a polymeric article,optionally in the presence of a surface modifying electrical discharge,and charging the fluorinated article to produce an electret.

The fluorination process includes modifying the surface of the polymericarticle to contain fluorine atoms by exposing the polymeric article toan atmosphere that includes fluorine containing species. Thefluorination process can be performed at atmospheric pressure or underreduced pressure. The fluorination process is preferably performed in acontrolled atmosphere to prevent contaminants from interfering with theaddition of fluorine atoms to the surface of the article. The atmosphereshould be substantially free of oxygen and other contaminants.Preferably the atmosphere contains less than 0.1% oxygen.

The fluorine containing species present in the atmosphere can be derivedfrom fluorinated compounds that are gases at room temperature, becomegases when heated, or are capable of being vaporized. Examples of usefulsources of fluorine containing species include, fluorine atoms,elemental fluorine, fluorocarbons (e.g., C₅F₁₂, C₂F₆, CF₄, andhexafluoropropylene), hydrofluorocarbons (e.g., CF₃H), fluorinatedsulfur (e.g., SF₆), fluorinated nitrogen (e.g., NF₃), fluorochemicalssuch as e.g., CF₃OCF₃ and fluorochemicals available under the tradedesignation Fluorinert such as, e.g., Fluorinert FC-43 (commerciallyavailable from Minnesota Mining and Manufacturing Company, Minnesota),and combinations thereof

The atmosphere of fluorine containing species can also include an inertdiluent gas such as, e.g., helium, argon, nitrogen, and combinationsthereof.

The electrical discharge applied during the fluorination process iscapable of modifying the surface chemistry of the polymeric article whenapplied in the presence of a source of fluorine containing species. Theelectrical discharge is in the form of plasma, e.g., glow dischargeplasma, corona plasma, silent discharge plasma (also referred to asdielectric barrier discharge plasma and alternating current (“AC”)corona discharge), and hybrid plasma, e.g., glow discharge plasma atatmospheric pressure, and pseudo glow discharge. Preferably the plasmais an AC corona discharge plasma at atmospheric pressure. Examples ofuseful surface modifying electrical discharge processes are described inU.S. Pat. Nos. 5,244,780, 4,828,871, and 4,844,979.

Another fluorination process includes immersing a polymeric article intoa liquid that is inert with respect to elemental fluorine, and bubblingelemental fluorine gas through the liquid to produce a surfacefluorinated article. Examples of useful liquids that are inert withrespect to fluorine include perhalogenated liquids, e.g., perfluorinatedliquids such as Performance Fluid PF 5052 (commercially available fromMinnesota Mining and Manufacturing Company). The elemental fluorinecontaining gas that is bubbled through the liquid can include an inertgas such as, e.g., nitrogen, argon, helium, and combinations thereof.

Charging the polymeric article to produce an electret can beaccomplished using a variety of techniques, including, e.g.,hydrocharging, i.e., contacting an article with water in a mannersufficient to impart a charge to the article, followed by drying thearticle, and DC corona charging. The charging process can be applied toone or more surfaces of the article.

One example of a useful hydrocharging process includes impinging jets ofwater or a stream of water droplets onto the article at a pressure andfor a period sufficient to impart a filtration enhancing electret chargeto the web, and then drying the article. The pressure necessary tooptimize the filtration enhancing electret charge imparted to thearticle will vary depending on the type of sprayer used, the type ofpolymer from which the article is formed, the type and concentration ofadditives to the polymer, and the thickness and density of the article.Pressures in the range of about 10 to about 500 psi (69 to 3450 kPa) aresuitable. An example of a suitable method of hydrocharging is describedin U.S. Pat. No. 5,496,507 (Angadjivand et al.).

The jets of water or stream of water droplets can be provided by anysuitable spray device. One example of a useful spray device is theapparatus used for hydraulically entangling fibers.

Examples of suitable DC corona discharge processes are described in U.S.Pat. No. Re. 30,782 (van Turnhout), U.S. Pat. No. Re. 31,285 (vanTurnhout), U.S. Pat. No. Re. 32,171 (van Turnhout), U.S. Pat. No.4,375,718 (Wadsworth et al.), U.S. Pat. No. 5,401,446 (Wadsworth etal.), U.S. Pat. No. 4,588,537 (Klasse et al.), and U.S. Pat. No.4,592,815 (Nakao).

The fluorinated electrets formed by the methods described herein aresuitable for use as, e.g., electrostatic elements in electro-acousticdevices such as microphones, headphones and speakers, fluid filters,dust particle control devices in,. e.g., high voltage electrostaticgenerators, electrostatic recorders, respirators (e.g., prefilters,canisters and replaceable cartridges), heating, ventilation, airconditioning, and face masks.

The invention will now be described further by way of the followingexamples.

EXAMPLES

Test Procedures

Test procedures used in the examples include the following.

Method for Determining CF₃:CF₂

ESCA data was collected on a PHI 5100 ESCA system (Physical Electronics,Eden Prairie, Minn.) using a non-monochromatic MgKα x-ray source and a45 degree electron takeoff angle with respect to the surface. The carbon(1 s) spectra were peak fit using a nonlinear least-squares routinesupplied by PHI (Physical Electronics, Eden Prairie, Minn.). Thisroutine used a linear background subtraction, and a gaussian peak shapefor the component peaks. The spectra were referenced to the hydrocarbonpeak at 285.0 eV. The CF₃ and CF₂ components were identified as thepeaks located at about 294 eV and 292 eV respectively (according to theprocedure described in Strobel et al., J. Polymer Sci. A: PolymerChemistry, Vol. 25, pp. 1295-1307. (1987)). The CF₃:CF₂ ratio representthe ratio of the peak areas of the CF₃ and CF₂ components.

Initial Dioctylphthalate Penetration (DOP) and Pressure Drop TestProcedure

Initial DOP penetration is determined by forcing 0.3 micrometer diameterdioctyl phthalate (DOP) particles at a concentration of between 70 and140 mg/m³ (generated using a TSI No. 212 sprayer with four orifices and30 psi clean air) through a sample of filter media which is 4.5 inchesin diameter at a rate of 42.5 L/min (a face velocity of 6.9 centimetersper second). The sample is exposed to the DOP aerosol for 30 secondsuntil the readings stabilize. The penetration is measured with anoptical scattering chamber, Percent Penetration Meter Model TPA-8Favailable from Air Techniques Inc.

Pressure drop across the sample is measured at a flow rate of 42.5 L/min(a face velocity of 6.9 cm/sec) using an electronic manometer. Pressuredrop is reported in mm of water (“mm H₂O”).

DOP penetration and pressure drop are used to calculate the qualityfactor “QF” from the natural log (ln) of the DOP penetration by thefollowing formula:${{QF}\lbrack {1\text{/}{mm}\quad H_{2}O} \rbrack} = \frac{{- {Ln}}\frac{{DOPPenetration}(\%)}{100}}{{PressureDrop}\lbrack {{mm}\quad H_{2}O} \rbrack}$

A higher initial QF indicates better initial filtration performance. Adecreased QF effectively correlates with decreased filtrationperformance.

DOP Loading Test

DOP loading is determined using the same test equipment used in the DOPpenetration and pressure drop tests. The test sample is weighed and thenexposed to the DOP aerosol for at least 45 min to provide a minimumexposure of at least about 130 mg. DOP penetration and pressure drop aremeasured throughout the test at least as frequently as once per minute.The mass of DOP collected is calculated for each measurement intervalfrom the measured penetration, mass of the filter web, and total mass ofDOP collected on the filter web during exposure (“DOP Load”).

Corona Fluorination

Example 1

A blown polypropylene microfiber web prepared from Exxon 3505Gpolypropylene resin (Exxon Corp.) and having an effective fiber diameterof 7.5 μm and a basis weight of 62 g/m² was prepared as described inWente, Van A., “Superfine Thermoplastic Fibers,” Industrial Eng.Chemistry, Vol. 48, pp. 1342-1346.

The blown microfiber web was then AC corona fluorinated in a 1% byvolume C₂F₆ in helium atmosphere at a corona energy of 34 J/cm², whichcorresponded to a corona power of 2000 W at a substrate speed of 1m/min. The AC corona fluorination treatment was performed in an ACcorona system that included the so-called “double-dielectric” electrodeconfiguration with a ground roll consisting of 40 cm diameternickel-plated aluminum roll covered with 1.5 mm of poly(ethyleneterephthalate) and maintained at a temperature of 23° C. usingrecirculating, pressurized water. The powered electrodes consisted of 15individual ceramic-covered electrodes (available from Sherman treatersLtd., Thame, United Kingdom) each with a 15 mm square cross-section andan active length of 35 cm. The electrodes were connected to a modelRS48-B (4 kW) variable-frequency power supply (available from ENI PowerSystems Inc., Rochester, N.Y.). The net power dissipated in the ACcorona was measured with a directional power meter incorporated into theENI supply. The frequency of the output power was manually adjusted toabout 16 kHz to obtain optimal impedance matching (minimum reflectedpower).

The AC corona system was enclosed within a controlled environment. Priorto treatment, the atmosphere surrounding the AC corona treatment systemwas purged with helium, and then continually flushed with 100 liters/minof 1% by volume C₂F₆ in helium, which was introduced near theelectrodes.

The microfiber web was taped onto a carrier film of 0.05 mm thickbi-axially-oriented polypropylene (BOPP), and then placed on the groundroll such that the carrier film was in contact with the ground roll,causing one side of the blown microfiber web to be exposed to thedischarge. After treatment, the blown microfiber web was flipped over,retaped to the carrier film, and AC corona treated a second time underthe same conditions as the first treatment to expose the other side ofthe blown microfiber web to the discharge.

Example 2

A G100 Filtrete fibrillated film web (available from Minnesota Miningand Manufacturing), having a basis weight of 100 g/m², was coronafluorinated following the method described in Example 1, with theexception that the ground roll was maintained at a temperature of 25° C.

Example 3

A polyethylene meltblown microfiber web, prepared from Aspun PE-6806polyethylene resin (DOW Chemical Company, Michigan) and having a basisweight of 107 g/m², was corona fluorinated following the methoddescribed in Example 2.

Example 4

A polyester staple fiber web (available from Rogers Corporation), havinga basis weight of 200 g/m², was corona fluorinated following the methoddescribed in Example 2.

Example 5

A poly-4-methyl-1-pentene meltblown microfiber web prepared from TPXMX-007 poly-4-methyl-1-pentene resin (Mitsui), and having a basis weightof 50 g/m² and an effective fiber diameter of 8.1 μm, was coronafluorinated following the method described in Example 2.

Examples 6-9

Examples 6-9 were prepared following the procedure in Example 1 exceptthat the source of fluorine containing species was as follows: 1% CF₄(Example 6), and 0.1% hexafluoropropylene (Example 7), 0.1% C₅F₁₂(Example 8), and 1.0% C₅F₁₂ (Example 9).

The surface chemistry of each of the sample webs of Examples 1-9 wasdetermined by ESCA analysis using a PHI 5100 ESCA system. The CF₃:CF₂ratio was determined for each of the samples of Examples 1-9 from theESCA data according to the above-described method. The results arereported in atomic % in Table I.

TABLE I Example Carbon Nitrogen Oxygen Fluorine CF₃:CF₂ 1 43 5.7 51 1.092 44 6.2 50 1.37 3 49 0.2 8.2 42 1.10 4 42 0.5 7.8 49 0.99 5 44 0.0 2.953 1.19 6 41 3.5 55 0.86 7 41 2.7 56 0.97 8 42 6.4 52 0.91 9 43 5.2 510.89

Hydrocharging

Example 10

A fluorinated polypropylene blown microfiber web prepared as describedabove in Example 1, was passed over a vacuum slot at a rate of 5 cm/sec(centimeters/second) while deionized water was sprayed onto the web at ahydrostatic pressure of about 90 psi from a pair of Spraying SystemsTeejet 9501 sprayer nozzles mounted 10 cm apart and centered 7 cm abovethe vacuum slot. The sample was then inverted and passed through thedeionized water spray a second time such that both sides of the web weresprayed with water. The deionized water spray was then removed and theweb was again passed over the vacuum slot to remove excess water. Theweb was then hung to dry at ambient conditions.

Example 11

A fluorinated poly-4-methyl-1-pentene meltblown microfiber web preparedaccording to Example 5 was charged following the procedure of Example10.

Examples 10A-11A

Examples 10A-11A were prepared following the procedures of Example 10and 11 respectively, with the exception that, after corona fluorinationand prior to hydrocharging, each of the fluorinated webs of Examples10A-11A were subjected to an anneal at 140° C. (300° F.) for about 10minutes.

Examples 13, 15, 16, 18 and 20

Examples 13, 15, 16, 18 and 20 were charged following the procedure ofExample 10, with the exception that the fluorinated polymeric fibrouswebs used in each of Examples 13, 15, 16, 18 and 20 were as follows: afluorinated polyethylene microfiber web prepared according to Example 3above (Example 13); a fluorinated polyester staple fiber web preparedaccording to Example 4 (Example 15); a fluorinated G100 Filtretefibrillated film web prepared according to Example 2 (Example 16); afluorinated polypropylene needle punched web (12 denier/fiber fibers ofExxon 3505 polypropylene resin), having a basis weight of about 200g/m², and having been corona fluorinated following the method describedin Example 1 (Example 18); and a polypropylene melt blown fine fiberweb, having a basis weight of 46 g/m² and an effective fiber diameter of3.7 μm, and having been corona fluorinated following the methoddescribed in Example 1 with the exception that 0.2% C₅F₁₂ was usedinstead of 1% C₂F₆ (Example 20).

DC Corona Charging

Example 12

The fluorinated polyethylene meltblown microfiber web of Example 3 wascharged using a DC corona discharge as follows. The fluorinated web wasplaced in contact with an aluminum ground plane, and then passed underan electrically positive DC corona source, in air, at a rate of about1.2 meters/min, while maintaining a current to ground plane of about0.01 mA/cm of corona source length. The distance from corona source toground was about 4 cm.

Examples 14, 17, 19

Examples 14, 17 and 19 were charged following the procedure of Example12, with the exception that the fluorinated polymeric fibrous webs foreach of Examples 14, 17 and 19 were as follows: a fluorinated polyesterstaple fiber web prepared following the procedure of Example 4 (Example14); a fluorinated polypropylene needle punched web (12 denier/fiberfibers made from Exxon 3505 polypropylene resin), having a basis weightof about 200 g/m², and having been corona fluorinated following themethod described in Example 1 (Example 17); and a fluorinatedpolypropylene meltblown fine fiber web, having a basis weight of 46 g/m²and an effective fiber diameter of 3.7 μm, and having been coronafluorinated following the method described in Example 1 with theexception that 0.2% C₅F₁₂ was used instead of 1% C₂F₆ (Example 19).

Examples 21-35

Examples 21-35 were prepared by fluorinating polypropylene blownmicrofiber webs following the procedure of Example 1, with the exceptionthat the source of fluorine for each of Examples 21-35 was as follows:1% CF₄ (Examples 21-23), 1% C₂F₆ (Examples 24-26), 0.1%hexafluoropropylene (Examples 27-29), 0.1% C₅F₁₂ (Examples 30-32), and1.0% C₅F₁₂ (Examples 33-35).

The fluorinated webs of Examples 23, 26, 29, 32, and 35 were thencharged following the hydrocharging process described above in Example10.

The fluorinated webs of Examples 22, 25, 28, 31 and 34 were then chargedfollowing the DC corona charging process described above in Example 12.

% DOP penetration (“%DOP PEN”), Pressure Drop (mmH₂O), and the QualityFactor (“QF”) for each of the electrets of Examples 10-35 weredetermined according to the above-described Initial DOP Penetration andPressure Drop Test Procedure. The results are summarized in Table II.

TABLE II PRESSURE EXAMPLE % DOP PEN DROP QF 10 0.119 3.65 1.84 10A 0.1403.21 2.05 11 2.45 1.46 2.54 11A 0.778 1.60 3.04 12 56.1 1.14 0.51 1338.1 1.15 0.84 14 78.3 0.38 0.64 15 65.6 0.41 1.03 16 27.3 0.40 3.25 1770.4 0.19 1.85 18 37.6 0.19 5.15 19 0.81 10.58  0.46 20 0.006 11.3 0.8621 55.6 2.83 0.21 22 15.0 3.28 0.58 23 0.288 3.09 1.89 24 54.1 3.05 0.2025 14.3 3.32 0.59 26 0.243 3.08 1.95 27 59.0 2.81 0.19 28 16.2 2.80 0.6529 0.276 2.90 2.03 30 52.5 3.15 0.20 31 14.0 3.11 0.63 32 0.250 2.992.00 33 45.3 3.10 0.26 34 14.9 2.93 0.65 35 0.244 3.14 1.92

Examples 36-39

Four flourinated, polypropylene microfiber webs were prepared accordingto Example 1 with the exception that the source of fluorine containingspecies was as follows: 0.1% hexafluoropropylene (“HFP”) (Examples 36and 38) and 0.1% C₅F₁₂ (Example 37 and 39).

Examples 36 and 37 further included charging the fluorinatedpolypropylene webs following the hydrocharging charging procedure ofExample 10.

Examples 38 and 39 further included charging the fluorinatedpolypropylene webs following the DC corona charging procedure of Example12.

Examples 36-39 were subjected to the above-described DOP Loading Test.The % DOP Penetration versus DOP loading (the amount of DOP collected onthe web in grams) for each of Examples 36-39 was measured according tothe above-described DOP Loading Test Procedure. The resulting data areplotted as % DOP penetration versus DOP load (grams) in FIGS. 1 and 2 asfollows: Examples 36 and 37 (indicated with x's and solid circlesrespectively) (FIG. 1), and Examples 38 and 39 (indicated with x's andsolid circles respectively) (FIG. 2).

Example 40

A 7 in. by 7 in. sample of polypropylene microfiber web having a basisweight of 61 g/m² was placed under a nitrogen atmosphere. A gaseousmixture of 5% by volume elemental fluorine diluted in nitrogen waspassed through the polypropylene microfiber web at a rate of 1.0 l/minfor 10 minutes. The fluorine concentration was then increased to 10% byvolume diluted in nitrogen and passed through the web at a rate of 1.0l/min for an additional 20 minutes.

The sample was then analyzed by ESCA and determined to have 62 atomic %fluorine and a CF₃:CF₂ ratio of 0.59, as determined according to theabove-described Method for Determining CF₃:CF₂.

The sample was then charged using a DC corona discharge as describedabove in Example 12, and subjected to the above-described DOP LoadingTest. The resulting data are plotted as % DOP Penetration versus DOPLoad (grams) in FIG. 3.

Example 41

A polypropylene blown microfiber web, having a basis weight of 20 g/m²and a web width of 15 cm, was vacuum glow-discharge treated in a C₅F₁₂environment. The glow-discharge treatment was performed in a vacuumchamber. The vacuum chamber contained a roll-to-roll glow dischargesystem consisting of an unwind roller, glow discharge electrodes, and awindup roller for the continuous treatment of the blown microfiber web.Two stainless steel electrodes were in the parallel plate configuration,each electrode was 20 cm wide and 33 cm long and they were separated bya gap of 2.5 cm. The top electrode was grounded and the bottom electrodewas powered by a 13.56 MHz rf generator (Plasma-Therm). The web traveledbetween the two electrodes and in contact with the top, groundedelectrode so that one side of the web was exposed to the discharge.

After loading the roll of blown microfiber web onto the unwind rollerunder C₅F₁₂ vapor at a pressure of 0.1 Torr. The blown microfiber webwas advanced through the electrodes at a speed of 17 cm/min to achievean exposure time to the plasma of 2 minutes. The discharge power was 50W. After the first side was treated, the chamber was vented and the webroll replaced onto the unwind roller to allow the other side of the webto be treated. The treatment of the second side of the web occurredunder the same conditions as the first side. After the fluorination,Example 41 was DC-corona charged following the process described abovein Example 12.

% DOP Penetration (“%DOP PEN”) for Example 41 was determined accordingto the above-described Initial DOP Penetration and Pressure Drop TestProcedure. The results are summarized in Table III.

TABLE III % DOP Penetration Loading Time (min) Example 14 0.5 28 10 28

Other embodiments are within the following claims. Although the electrethas been described with reference to nonwoven polymeric fibrous webs,the electret can be a variety of polymeric articles including, e.g.,those polymeric articles described in U.S. patent application Ser. No.09/106,506, entitled, “Structured Surface Filter Media,” (Insley etal.), filed on Jun. 18, 1998.

All of the patents and patent applications cited above are incorporatedby reference into this document in total.

What is claimed is:
 1. A method of making an electret, said methodcomprising fluorinating a nonwoven web, said fluorinating comprisingexposing said web to an electric discharge in the form of a plasmacomprising fluorine containing species for a period sufficient to modifythe surface of the web such that the surface includes fluorine moieties,and charging the fluorinated web to impart an electret charge to theweb, the resulting electret, when tested according to the Initial DOPPenetration Test and the DOP Loading Test, exhibiting a DOP penetrationof less than 20% for a DOP load from 0.05 grams to 0.2 grams.
 2. Themethod of claim 1, wherein the electret exhibits a DOP penetration ofless than 15% for a DOP load of from 0.05 grams to 0.2 grams.
 3. Themethod of claim 1, wherein the electret exhibits a DOP penetration ofless than 10% for a DOP load from 0.05 grams to 0.2 grams.
 4. The methodof claim 1, wherein the electret exhibits a DOP penetration of nogreater than 5% for a DOP load from 0.05 grams to 0.2 grams.
 5. Themethod of claim 1, wherein the electret exhibits a DOP penetration ofless than 10% for a DOP load of from 0.02 grams to 0.08 grams.
 6. Themethod of claim 1, comprising fluorinating the web such that the webcomprises at least 45 atomic % fluorine as detected by ESCA.
 7. Themethod of claim 1, comprising fluorinating the web such that the webcomprises a CF₃:CF₂ ratio of at least 0.45 as determined according tothe Method of Determining CF₃:CF₂.
 8. The method of claim 1, comprisingfluorinating the web such that the web comprises at least 45 atomic %fluorine as detected by ESCA and a CF₃:CF₂ ratio of at least 0.45 asdetermined according to the Method of Determining CF₃:CF₂.
 9. The methodof claim 1, comprising fluorinating the web such that the web comprisesa surface fluorination of at least 50 atomic % fluorine as detected byESCA.
 10. The method of claim 1, comprising fluorinating the web suchthat the web comprises at least 50 atomic % fluorine as detected by ESCAand a CF₃:CF₂ ratio of at least 0.25 as determined according to theMethod for Determining CF₃:CF₂.
 11. The method of claim 1, comprisingfluorinating the web such that the web comprises at least 50 atomic %fluorine as detected by ESCA and a CF₃:CF₂ ratio of at least 0.45 asdetermined according to the Method for Determining CF₃:CF₂.
 12. Themethod of claim 1, comprising fluorinating the web such that the webcomprises a CF₃:CF₂ ratio of at least 0.9.
 13. The method of claim 1,comprising fluorinating the web such that the web comprises at least 50atomic % fluorine as detected by ESCA and a CF₃:CF₂ ratio of at least0.9 as determined according to the Method for Determining CF₃:CF₂. 14.The method of claim 1, wherein the electret exhibits a Quality Factor ofat least 0.25/mmH₂O.
 15. The method of claim 1, wherein the electretexhibits a Quality Factor of at least 0.5/mmH₂O.
 16. The method of claim1, wherein the electret exhibits a Quality Factor of at least 1/mmH₂O.17. The method of claim 1, wherein the electret exhibits a QualityFactor of at least 2/mmH₂O.
 18. The method of claim 1, wherein the webcomprises fibers selected from the group consisting of polycarbonate,polyolefin, polyester, halogenated polyvinyl, polystyrene, or acombination thereof.
 19. The method of claim 1, wherein the webcomprises fibers selected from the group consisting of polypropylene,poly-(4-methyl-1-pentene), or a combination thereof.
 20. The method ofclaim 1, wherein the web comprises meltblown microfibers.
 21. The methodof claim 20, wherein the microfibers comprise polyolefin.
 22. The methodof claim 20, wherein the microfibers comprise polypropylene.
 23. Themethod of claim 1, wherein the web has a basis weight of from 10 to 100g/m².
 24. The method of claim 1, wherein the web has a basis weight offrom 10 to 100 g/m².
 25. The method of claim 1, wherein the web has athickness of 0.25 to 20 mm.